1. Field of the Invention
This invention relates to a cooling system, especially to a cooling system using a turbo compressor and a turbo expander. In this kind of cooling system, the temperature goes down below 200 K and the cooling system is used for liquefying nitrogen, hydrogen or helium, and cooling an infrared sensor or processors for a super computer or a cryopump.
2. Description of Related Art
A conventional cooling system provides a reciprocating or screw type compressor and a turbo expander. In this cooling system, a compressor compresses a coolant such as helium and a turbo expander expands the coolant. On the other hand, there is also known a cooling system using a turbo compressor and a turbo expander as shown in FIG. 5. This cooling system, however, has not been commercialized because it is difficult to supply all the flow of coolant from the compressor to the expander.
Referring to FIG. 5, this system provides a turbo compressor 114 and a turbo expander 115. The turbo compressor includes impellers 101 and 102 driven by a high speed motor 103. The impeller 101 compresses a coolant such as nitrogen, neon or helium and the coolant goes to a radiator 104. The heat of the coolant caused by the compression is then removed by a cooling fluid such as water or air flowing through a passage 112. Then the coolant is again compressed by the impeller 102 and goes to the radiator 105. The heat of the coolant caused by the second compression step is also removed by a cooling fluid such as water or air flowing through the passage 112 in the radiator 105.
The coolant then goes into the turbine 107 of the expander 115 via the heat exchanger 106. The coolant is cooled in the heat exchanger 106 by transferring heat to a reverse flowing coolant in the heat exchanger 106. The coolant expands in the turbine 107 and drives the turbine 107 and the high speed generator 108 connected thereto. The expanded coolant is thus at a low temperature.
The expanded coolant then goes into a cooler 109 where it cools down the cooled object 110. Then the coolant goes back to the impeller 107 via the heat exchanger 106 while cooling down the coolant flowing the opposite way in the heat exchanger 106. By this cycle, a temperature in object 110 can be lowered to from -70.degree. C. to -270.degree. C.
FIG. 2 shows a mass flow rate of the coolant versus an expansion ratio at the turbine 107 of the turbo expander 115. When the volume flow rate of the coolant increases, an expansion ratio also increases since the area of the turbine 107 is constant. Similarly, if the temperature is constant, an expansion ratio increases when a mass flow ratio (g/sec) of the coolant increases. At a low temperature such as 100 K (-73.degree. C.), as shown by the solid line in FIG. 2, a mass flow is ml at the steady state pressure r1 at the steady state point A.
However, when the cooling system is started at room temperature (dash lines in FIG. 2), a temperature of the coolant at the turbine 107 is about room temperature, e.g., 300K (27.degree. C.). The volume of the coolant is proportional to absolute temperature, so that the same mass flow as at -73.degree. C. now causes about a triple volume of the flow as compared to flow at -73.degree. C. This increases the expansion ratio to r1', which is bigger than the steady ratio r1. In other words, at the steady state point B a mass flow of the coolant at room temperature for an expansion ratio r1 is m0, which is less than that of the coolant at a lower temperature.
FIG. 3 shows a graph of the coolant as compressed by both the impellers 101 and 102. By ignoring the pressure loss at the radiators 104 and 105 and the heat exchanger 106, an expansion ratio at the turbine 107 is equal to the sum of the compression ratios of the impellers 101 and 102. The impellers 101 and 102 have a surge line as shown in FIG. 3 and if the operation is conducted above this surge line, vibration of the coolant makes the system inoperative. At the steady point D of the turbo compressor 114 with a revolution speed of N1, a mass flow is ml and a compression ratio is r1.
At the beginning of the operation, a compression ratio is r1' shown by point C in FIG. 2. If all the mass flow m1 is sent to the turbine 107, this causes a surge since r1' is much higher than r1. On the other hand, at the beginning of operation a mass flow applied to the turbine 107 at the compression ratio r1 can be as low as a mass m0 as shown by a point B in FIG. 2. In order to reduce the mass flow to m0 at this time, the revolution speed may be reduced to N0. However, a compression ratio r2 which causes a surge goes down to the point F at such low rotational speeds. Therefore, a surge is caused if a high compression ratio of r1 is applied at low rotational speed N0 (point E in FIG. 3).
As explained above, this coolant system could work if a temperature of the coolant is low enough at the turbine 107. However, the coolant system cannot start at room temperature because of surging.